1. Field of the Invention
The present invention is directed to a shim tray for holding laminae of a passive shim system for a magnetic resonance apparatus. The invention also is directed to a gradient coils system for accepting such a shim tray, as well as to a magnetic resonance apparatus for accepting such a shim tray.
2. Description of the Prior Art
Magnetic resonance technology is a known technology for, among other things, acquiring images of the inside of the body of an examination subject. In a magnetic resonance apparatus, rapidly switched gradient fields that are generated by a gradient coil system are superimposed on a static basic magnetic field that is generated by a basic field magnet. The magnetic resonance apparatus also has a radio-frequency system that emits radio-frequency signals into the examination subject for triggering magnetic resonance signals and picks up the magnetic resonance signals, on the basis of which magnetic resonance images are produced.
A high homogeneity of the basic magnetic field is an important factor for the quality of the magnetic resonance images. Inhomogeneities of the basic magnetic field within a homogeneity volume of the magnetic resonance apparatus cause geometrical distortions of the magnetic resonance image that are proportional to the inhomogeneities. The fat-water separation is also important in this context. An RF pulse without gradient activation is emitted at the frequency of the hydrogen in the fat. The protons excited in this way only slowly return back into the field direction and therefore cannot be re-excited for a certain time. Images acquired during this time thus shown no fat but only water. To this end, however, the homogeneity in the imaging volume must be better than 1 ppm peakxe2x80x94peak since the distance of the fat line from the water line amounts to approximately 3 ppm. Arrangements referred to as shim systems are employed for improving the basic magnetic field homogeneity within the homogeneity volume. A distinction is made between passive and active shim systems. In a passive shim system, a number of laminae composed of a magnetic material, particularly a ferromagnetic iron alloy, are attached in the examination space of the magnetic resonance apparatus in a suitable arrangement. To that end, the basic magnetic field is measured within the homogeneity volume before the attachment of the laminae. Using the measured values, a computer program determines the suitable number and arrangement of the laminae.
For example, U.S. Pat. No. 5,635,839 discloses a hollow-cylindrical gradient coil system of a magnetic resonance apparatus that has shim receptacles with a cross section having the shape of an annular segment, that are continuous in the axial direction. Corresponding shim trays are thereby insertable into the shim tray receptacles, the length of the shim trays being equal to a length of the gradient coil system and the shim trays being subdivided into corresponding shim pockets for the acceptance of ferromagnetic laminae.
Further, for example, U.S. Pat. No. 5,786,695 discloses that the introduction and removal of shim trays filled with shim laminae into or out of shim tray receptacles must ensue with the basic magnetic field turned off according to regulations. If the introduction and removal were to be undertaken inadvertently with the basic magnetic field turned on, then the shim tray disclosed in U.S. Pat. No. 5,786,695 is fashioned such that a cover of the shim tray is secured against the forces at the rest of the shim tray acting on the shim laminae.
Particularly in the case of a magnetic resonance apparatus having a tunnel-shaped examination space, a support mechanism can be introduced into and removed from the examination space through a first opening of the end of the space. Adequate space adjoining the first opening must be provided for an installation room of the magnetic resonance apparatus, so that a complete removal of the support mechanism from the examination space can be unproblemmatically implemented. By contrast, the magnetic resonance apparatus can be constructed with a second opening of the examination space lying opposite the first opening that has a comparatively small distance from a limiting surface of the installation room. In many instances, this prevents installation of shim trays that, corresponding to the length of the gradient coil system, have a length of one meter or more proceeding from a side of the second opening. So that the shim trays can be introduced regardless of the installation condition, the magnetic resonance apparatus together with the movable support mechanism usually is designed such that introduction is possible proceeding from the side of the first opening.
An object of the present invention is to provide a shim tray, a gradient coil system and a magnetic resonance apparatus for the acceptance of the shim tray such that, among other things, a maximized volume is available for shimming.
This object is achieved by a shim tray fashioned with a cross-section corresponding to an annular segment for insertion into a shim tray receptacle, and wherein for the acceptance of shim laminae, the shim tray has at least one shim pocket having a cross-section corresponding to a rectangle, and wherein the shim tray is fashioned such that, for achieving a maximum area content of the rectangle, the rectangle is arranged within the annular segment such that a circumferential line of the rectangle touches a circumferential line of the annular segment in exactly three points.
As a result and compared to conventional shim tray receptacles and shim trays, either a greater maximum shim effect can be achieved given the same available space for shim tray receptacles, or the space required for shim tray receptacles can be reduced given the same maximum shim effect.
In an embodiment, the shim box is divided in a direction of insertion into at least two shim sub-boxes that can be released from one another. As a result thereof, an insertion and removal of the sub-boxes filled with shim laminae into or out of the shim box receptacle can be implemented as a normal procedure when the basic magnetic field to be shimmed is activated. The reason for this is that, compared to a comparable, undivided shim box, the magnetic forces acting on the sub-boxes only amount to a fraction. Advantageously, thus, a shut-down of the basic magnetic field is not necessary for the introduction and removal of the sub-boxes. The introduction and removal of the sub-boxes can be implemented with a comparatively slight exertion of force and can be supported by a suitably designed device. With the basic magnetic field activated, further, the sub-boxes filled with shim laminae can be carried or transported past a basic field magnet generating the basic magnetic field in the influencing region of the basic magnetic field, for example at a distance of approximately 50 cm. As a result, a general risk of accidents is reduced. Compared to a comparable, undivided shim box, further, the space required for the introduction and removal of the sub-boxes is significantly reduced, for example a distance between an opening of the shim box receptacle and a wall of a shielded compartment. Due to the shortness of the sub-boxes, further, lower tool costs are incurred in the manufacture, and the sub-boxes can be manufactured with a higher precision.